1. Field of the Invention
The present invention relates to a method for measuring the surface configuration of a workpiece by a contact-type probe attached to a coordinates measuring machine, etc. More specifically, the present invention relates to improvement in measurement efficiency and accuracy with use of contact detection probe vibrated by ultrasonic wave.
2. Description of Related Art
A height gauge (one-dimensional measuring machine), a coordinates measuring machine, and a profile measuring machine are known for measuring the configuration and/or dimension of a workpiece. Various probes are used by these measuring machines in order to detect the positional relation between the measuring machine and the workpiece. The probes are classified into non-contact type probes and contact type probes, continuously measuring probes, and contact detection probes (touch trigger probes), etc.
A touch signal probe vibrated by ultrasonic waves disclosed in Japanese Patent Laid-Open Publication No. Hei 6-221806 is known as a contact detection probe used for a coordinates measuring machine. The contact type vibrating probe disclosed in this publication has a stylus having a contact portion to be in contact with a workpiece at a tip end thereof, a stylus holder for supporting the stylus, a vibrator for resonating the stylus in an axial direction thereof by applying ultrasonic vibration, and a detector for detecting a change in the stylus""s vibration by the vibrator.
According to the touch signal probe, since the vibration status of the stylus changes when the tip end of the stylus touches a surface of the workpiece, the surface position of the workpiece can be detected by the detector.
Such a touch signal probe vibrated by an ultrasonic wave is sometimes used for measuring the diameter of a small hole etc. For measuring small holes, another touch signal probe shown in U.S. patent application Ser. No. 09/366,774 has been proposed as a small size touch signal probe vibrated by ultrasonic wave.
As shown in FIG. 12, the touch signal probe 100 has a stylus holder 101, a stylus 102, a vibrator 103A, and a detector 103B. A contact portion 102A to be in contact with the workpiece is provided at an end of the stylus 102 and a counter balance 102B is provided at a base end of the stylus 102. When the stylus 102 vibrates in an axial direction thereof, the centroid position becomes a node of vibration.
In the touch signal probe 100, the stylus 102 is composed of a thin stick member and the contact portion 102A composed of a small sphere in line with the stylus 102 to enable small hole measurement. Further, since the thin stylus 102 is difficulty to support at one point, the stylus holder 101 supports the stylus 102 at two points sandwiching the centroid position of the stylus 102.
The vibrator 103A and the detector 103B are made by dividing a piezoelectric element 103 stretching over the two supporting portions of the stylus holder 101. When the stylus 102 resonates along the axial direction by the vibrator 103A, the node of vibration is caused at the centroid position of the stylus 102. The supporting portions of the stylus 102 of the stylus holder 101 sandwich the node of vibration.
According to the touch signal probe 100, since the stylus holder 101 supports the stylus 102 at the two portions sandwiching the node of vibration, the stylus 102 can be supported by the stylus holder 101 even when the stylus 102 is made from an extremely thin stick member, thus enabling the inner surface measurement of a small hole having a large aspect ratio.
On the other hand, another method for touching the surface of the workpiece by vibration movement (xe2x80x9ctapping methodxe2x80x9d) has been proposed (e.g. U.S. patent application Ser. No. 09/540,051).
According to the above method, a second vibrator is provided to, for instance, the above-described stylus holder 101 and the stylus 102 is vibrated by the second vibrator, so that the contact portion 102A at the tip end vibrates toward and away from the workpiece.
According to this tapping method, even when the stylus 102 moves along the surface of the workpiece to measure the surface configuration continuously, since the contact portion 102A moves toward and away from the workpiece, dragging (adhesion phenomenon) by the workpiece can be avoided even with the stylus 102 having less rigidity. Therefore, the method is advantageous in that work efficiency can be improved by the continuous measurement while maintaining high accuracy.
However, measurement accuracy is largely influenced by a timing for outputting trigger of contact detection in the above-described touch signal probe vibrated by ultrasonic wave.
Specifically, when the stylus vibrated by the vibrator moves toward the workpiece and the contact portion touches the surface of the workpiece, vibration starts to be restrained when contact is initiated. When the contact portion further moves toward the workpiece and the contact portion touches the surface of the workpiece with a certain pressing force, the vibration of the stylus is further restrained. At this time, since the pressing condition toward the workpiece surface and restraint of the vibration are interrelated, the force of the stylus toward the workpiece can be made constant by sensing that the restraint of the vibration reaches a predetermined level by monitoring the output signal of the detector, so that accurate position detection is possible.
However, for measuring a position of one point on the surface of the workpiece, it is necessary to repeatedly move the stylus toward and away from the workpiece detecting when the vibration of the stylus is settled at the position. An enormous total amount of work time may be required for checking numerous points when, for instance, continuously measuring the surface of the workpiece.
On the other hand, the work time can be shortened for measuring respective positions of the workpiece if the vibration of the stylus is not strictly settled at the position but, settled only to a predetermined level. However, in this case, measurement accuracy naturally deteriorates.
An object of the present invention is to provide a surface configuration measuring method for avoiding the influence of convergence time at each contact point on the workpiece while detecting contact by vibration, thereby improving work efficiency while maintaining high measurement accuracy.
The present invention focuses on the fact that, in the conventional contact detection by vibration, the stylus is moved to a position where the vibration of the stylus reaches a predetermined level, and position information of the stylus at the position is read to obtain contact position. The time to take the troublesome steps for minutely moving the stylus toward and away from the workpiece is known as the convergence time. In the present invention, the stylus is not settled to a predetermined position, but a coordinate of the predetermined position conventionally to be settled is calculated by the detected value of the detector obtained adjacent to the predetermined position and the detected position of the stylus, thus measuring the position of the surface of the workpiece with high accuracy and efficiency.
More specifically, the present invention is a surface configuration measuring method for measuring a surface configuration of a workpiece using a contact detection probe comprising: a support body for moving in a three-dimensional space by a predetermined command velocity vector based on an outside command; a stylus being supported by the support body and having a contact portion to be in contact with the workpiece; a vibrator for resonating the stylus at a first frequency f1 in an axial direction; and a detecting circuit for detecting change in vibration of the stylus by the vibrator, the surface configuration of the workpiece being measured by a position of the support body when the contact portion touches the surface of the workpiece,
the surface configuration measuring method being characterized in having the steps of:
moving the contact detection probe by the command velocity vector to touch the surface of the workpiece to be measured;
scanning the surface of the workpiece to be measured, wherein the contact detection probe is moved along the surface to be measured while controlling the distance relative to the surface to be measured so that the detected amplitude value An of a detection signal outputted by the detecting circuit becomes a predetermined reference value As, thus outputting the detected amplitude value An and corresponding measuring position rn; and
calculating estimated surface position Rn from a set of the detected amplitude value A0xcx9cAm and a set of the measuring position r0xcx9crm.
As the contact detection probe, the above-described touch signal probe vibrated by ultrasonic wave, etc. can be used. The stylus may preferably be held by a stylus holder connected to the support body. An existing driving element, such as piezoelectric element, can be used as the vibrator. A power source or driving circuit may be connected to an outside source for actuating the vibrator. The detector may be composed of a piezoelectric element and the existing structure integrated with the vibrator, etc. may be used as necessary.
The support body may be driven depending upon any existing coordinates measuring machine and the support body may be operated based on a command program executed by an outside controller or a computer system. The outside controller or the computer system may collect the detected amplitude value An from the detector, control in accordance with the reference value As and calculate the estimated surface position Rn.
Collection of detected amplitude value An and the measuring position rn and calculation of the estimated surface position Rn may be conducted in parallel, or alternatively, may be sequentially conducted.
Conducting the steps in parallel means that, while detecting the position of rn, the estimated surface position for the previous several steps is calculated based on the detected amplitude value and the measuring position.
According to the above arrangement, after the stylus touches the surface of the workpiece and the detected amplitude value An, and the measuring position rn of respective points are accumulated by scanning the surface, based on which the estimated surface position Rn is calculated.
At this time, though the stylus is controlled to keep the detected amplitude value An at the level of the reference value As for respective points, and the stylus does not stay on the respective points until the detected amplitude value An is settled into the reference value As, so that conventional convergence time is not required.
On the other hand, since the estimated surface position Rn can be obtained by correcting the measuring position rn with the detected amplitude value An, an accurate value can be obtained even when the stylus does not reach the position where the detected amplitude value An equals to the reference value As.
In the present invention, the command velocity vector Vn+1 during the scanning step may preferably be determined by equalizing a scalar of the vector product of the preceding value of the command velocity vector Vn and a current command velocity vector Vn+1 with a difference between the detected amplitude value An and the reference value As multiplied by a predetermined proportionality factor k.
Accordingly, the stylus can be easily and appropriately controlled to move so that the detected amplitude value An approximates the reference value As.
In the present invention, the estimated surface position Rn calculated during the calculating step may preferably be a position determined by, after determining succeeding measuring position rn+1 relative to measuring position rn, setting a position starting from the measuring position rn having magnitude corresponding to the detected amplitude value An and being corrected orthogonal with a straight line connecting the succeeding measuring position rn+1 and the preceding measuring position rn+1.
Accordingly, the estimated surface position Rn can be obtained by an easy calculation and with high accuracy.
Further, the estimated surface position Rn calculated during the calculating step may be defined as a position starting from the measuring position rn having magnitude corresponding to the detected amplitude value An and being corrected orthogonal with a straight line connecting the measuring position rn and the preceding measuring position rnxe2x88x921.
Accordingly, since the succeeding measuring position rn+1 is not required, the detection data can be immediately arithmetically processed.
Further, the estimated surface position Rn calculated during the calculating step may be defined as a position starting from the measuring position rn having a magnitude corresponding to the detected amplitude value An and being corrected in a direction of a perpendicular drawn to a curve defined at least by three points of the measuring position rn and preceding and succeeding measuring positions rn+1, rnxe2x88x921.
According to the above arrangement, the accuracy of the estimated surface position Rn can be further improved.
In the present invention, the estimated surface position Rn may preferably be determined by adding the offset amount D of the contact portion of the stylus relative to the stylus axis with a position corresponding to the detected amplitude value An.
The offset amount D is, when the contact portion of the stylus is spherical for instance, given as the radius of the contact portion etc., which means that the distance of the actual contact position is added to the position of the stylus axis.
In the present invention, the scanning step of the surface to be measured may preferably be initially completed and all the necessary detected amplitude values An and corresponding measuring positions rn may be sequentially stored during the scanning step, and a set of estimated surface positions R0xcx9cRm may be obtained based on the stored set of detected values A0xcx9cAm and set of corresponding measuring values r0xcx9crm during the above calculation step. For sequentially storing the detected amplitude value An and the corresponding measuring position rn, the above-described outside computer system etc. may be used as necessary.
According to the above arrangement, since the scanning step for detecting the detection values A0xcx9cAm and the measuring positions r0xcx9crm can be independently and sequentially conducted, the control can be simplified as compared to conducting both steps in parallel. Further, since it is not necessary to consider the operation timing of both steps, the operation of respective steps can be adjusted to be the fastest.
In the present invention, the contact detection probe may further have a second vibrator for resonating the stylus crosswise relative to an axis thereof at a second frequency f2, and the detected amplitude value An of the detection signal outputted by the detecting circuit may be latched when vibration of the second vibrator is at a predetermined phase and the latched detected amplitude value An may be compared with the reference value As to be controlled.
Accordingly, the present invention can be applied to the above-described tapping method.
At this time, the stylus may preferably be vibrated by the second vibrator in a direction approximately orthogonal with the axis of the stylus and approximately orthogonal with moving direction of the support body during the scanning step.
Accordingly, when the stylus axis is parallel to a wall surface, the stylus can tap always in approximately perpendicular direction relative to the measurement surface of the workpiece.
In the present invention, the support body may have a tilting driver for rotating and tilting the contact detection probe in a desired attitude and the attitude of the contact detection probe may be controlled in accordance with the configuration of the workpiece by a tilting controller.
Accordingly, the measurement surface of the workpiece and the axis of the contact detection probe can be made parallel, so that the measurement according to the present invention can be easily conducted even when the measurement surface of the workpiece is slanted.
In the above, the tilting driver may preferably be controlled by the tilting controller so that the stylus is vibrated by the second vibrator in the normal direction of the surface of the workpiece to be in contact with the contact portion.
Accordingly, even when the measurement surface of the workpiece is slanted, accurate measurement is possible by tapping in the normal direction.
Incidentally, for adjusting the attitude of the contact detection probe, information on the measurement surface of the workpiece is necessary. Such information may be obtained by measuring position of a plurality of points on the measurement surface of the workpiece in advance, or by utilizing design data, for instance, CAD data of the workpiece.
In the present invention, the stylus may preferably be linearly vibrated by the second vibrator.
Accordingly, suitable tapping operation can be conducted in the present invention.